Process for segregating powdered materials into fractions of different particle size



Patented Aug. -7, 1951 PROCESS FOR SEGREGATING POWDERED MATERIALS INTO FRACTIONS OF DIFFER- ENT PARTICLE SIZE Herman Verschoor, Amsterdam, Netherlands, as-

slgnor to Shell Development Company, San Francisco, Calii.,a corporation oi. Delaware No Drawing. Application June 14, 1948, Serial No. 32,987. In the Netherlands June 27, 1947 l 12 Claims. The invention relates to a process of segregating a mixture of powdered particles of different sizes into fractions wherein relatively finer and coarser particles are concentrated, such process involving the formation of conglomerates of the finer particles by solely mechanical means.

The practical application of powdered materials, for instance, in the paint industry, in the preparation of pharmaceutical and toilet articles, such as tooth-paste, skin-ointments, and catalysts for fluidized operations, and the like, makes it desirable to have the disposal of a material which, besides being homogeneous as to size of the particles, is also very finely divided.

With the known process it is possible to segregate powdered material into fractions according to the sizes of the diiferent particles, which may be efiected in several manners. The usual methods to be applied comprise screening, wind-siftin and sedimentation.

In screening one is limited by the mesh width of the available screens, which renders it difllcult to separate from the finest fraction particles of the order of size of 50 micron diameter or less. Consequently, the sedimentation process has been often resorted to, using different settling velocities in accordance with the particle sizes dealt with. In some cases, however, this method is not practical, for instance, if the material cannot be wetted or if an expensive drying process following the separation is to be avoided.

The only remaining known method, viz., wind sifting, or air-classification, may be applied in many cases. With a number of powders, however, even this procedure does not yield the desired results. Experiments with magnesia powder and tannin, for example, have revealed that particles having diameters of the order of size of microns and less could not be separated from the powder.

It is an object of this invention to .provide a process for effecting a separation of flne particles from coarser particles by purely mechanical means which does not depend upon screening, sedimentation or wind sifting in the first instance, although these procedures may be employed in a subsequent step of the process.

It is a further object to provide an improved process for effecting a separation of the type described whereby the finest particles can be segregated from coarser particles without the addition of wetting or adhesive materials; and to provide a process whereby particles having diameters of the order of size less than 10 microns can be effectively segregated.

In accordance with the present invention it was found that powdered materials of greatly divergent types can be split into -fractions of various particle sizes by passing through a layer of the powdered material a gas at such a rate as to cause the particles to be fluidized; in this fluidized state the finer particles conglomerate to form conglomerates larger than the coarser particles. These conglomerates are then separated from the unconglomerated, coarser particles by any physical means, such as the known process previously described. This process is particularly effective in separating the fraction with the finest particles from the bulk of the powdered material.

As is well understood in the art of catalytic reactions, a fluidized state is one in which the particles are suspended in an ascending current of gas in a relatively dense layer of gas and particles, having a more or less defined and readily recognizable upper surface above which the ascending gas is for the greater part free from suspended solids. Such a fluidized layer resembles a dense cloud and behaves somewhat like a liquid. When the velocity of gas is increased above that suitable for the fluidized state the particles are carried upwardly with the gas and the upper surface disappears. The linear, upward gas velocity depends upon various factors, such as the particle sizes, the pressure and relative densities of the particles and the gas; velocities of 4 to 15 cm. per second are typical.

It should be particularly noted that the formation of conglomerates in the instant process is brought about by a treatment which is solely mechanical, that is to say, it does not depend upon the presence or addition of extraneous agents for causing agglomeration, such as tar and sticky materials. It is already known that fine particles of catalytic material will, under certain conditions, form larger balls (U. 8. Patent No. 2,355,016) which must be pulverized; there, however the conglomeration of fine particles is due to the presence of tar-like substances deposited on the catalyst during the cracking operation.

Moreover, the present separating process is carried out in such a manner that the chemical nature of the particles remains unchanged and the gas stream does not deposit a layer on the particles or remove any deposit.

The influence of moisture content of the gas does not appear to be significant, since it does not matter whether ordinary air or dried air is applied. Besides air, it is possible to use other available gas, such as nitrogen, hydrogen, gaseous hydrocarbons, etc., which should preferably 3 be inert with respect to the powdered material being treated.

The treatment may be applied to a wide variety of powdered materials, such as various inorganic metal oxides, like the oxides of aluminum and silica, including those suitable for use as catalysts in fluidized operations, magnesium oxide, tannin, chalk, quartz powder, etc. The particles of tannin and magnesium oxide tested in the following examples measured less than about 1 to 5 in diameter and these diameters are typical of the powdered particles to be treated by this process which may, however, also be applied to somewhat coarser and finer particles. In most instances, the finer particles which are conglomerated have particle diameters less than about The process according to the invention may, for example, be carried out in a vertical cylinder by passing a current 0! air upwardly through a quantity of powder contained in the cylinder. This results in an expansion of th powdered mass to form a fluidized layer as is the case with the formation of a fluid bed in catalyst technique. The fluidized layer should be maintained as a relatively high density, as in fluidized catalyst technique, so that mutual contact of the finest particles is ensured. These particles are thereby formed into more or less globular conglomerates collecting at the bottom of the cylinder.

As a rule these globules have diameters ranging between about 1 and 4 mm. After these conglomerates have been formed the supply of gas is stopped, causing the fluid bed to collapse. The resulting mixture of conglomerates and unconglomerated, coarser particles is then subjected to any physical separation treatment wherein the conglomerates are separated as the larger particles. For example, the mixture may be screened, using a mesh to retain the conglomerates. In most instances, the screen should be such as to retain. particles having diameters of about 14 mm. in diameter, and to pass finer particles, although the mesh of the screen will in each instance depend upon the conglomerate sizes. According to a modification, the flow of gas may be increased following conglomeration of the finer particles, so as to carry off the coarser, unconglomerated particles in suspension and leave the conglomerates as a residue.

The process may also be carried out continuously, by continuously or intermittently discharging the conglomerated globules from the bottom of the treating vessel as they are formed and during the continuing contact of powder remaining in the vessel. When, as is usually the case,

unconglomerated particles are also withdrawn, the withdrawn material may be subjected to a physical separation, e. g., screening; however, it is possible to withdraw the globules relatively free from other particles.

The process of the invention may be combined with one or more of the known methods for effecting the segregation of the particles according to size. Thus, the powdered material may be given a pre-treatment by one of the known methods for first separating therefrom the coarsest particles, and subjecting the remaining powder, e. g., consisting of particles smaller than about to 300 1. diameters, to the treatment according to the present invention.

0n the other hand, the process according to the invention may be carried out so that the very fine particles are obtained in two or more additional separate fractions, as by partially conglomerating the fine particles, removing the conglomerates as a first fine fraction. and then continuing the fluidized treatment to form further conglomerates, which may be separated as a second fine fraction, containing particles of average diameters somewhat greater than those of the first fine fraction.

In passing the gas through the fluidized bed of powdered material, it was found that after some time a state is reached in which no further conglomeration occurs. When the process is applied to the coarser, unconglomerated particles remaining after the separation of the conglomerates, no further conglomeration is observed, from which it appears that for each material conglomeration is dependent upon a certain particle size.

As before indicated, the instant process may be applied in the formation of a fluid bed in the catalyst technique. Although the phenomenon of conglomeration has been actually observed in such technique, it was regarded as an objectionable phenomenon not suitable for the purpose of this invention and steps were taken to prevent conglomeration.

Experiments have shown that on separating the conglomerated particles a powder is left with which an excellent fluid bed may be obtained. This has the advantage that powders which were heretofore regarded as unsuitable because of balling up may be rendered suitable for fluidized operations by a treatment according to the invention.

Emample I In a tube of diameter of 7 cm. and a height of 2 m. a quantity of tannin powder was introduced to a height of 50 cm. The maximum size of the particles of the powder amounted to 250 microns. Subsequently a current of air was passed upwards through the powder at a linear velocity of 10-15 cm./sec., based on the empty tube. The powder was thereby caused to move and expand to a height of 70 cm., directly followed by conglomeration of part of the powder. After approximately 20 minutes no further conglomeration occurred and the supply of air was shut ofi.

With the aid of a screen the total contents of the tube were then separated into a powder fraction and a screen residue which contained the conglomerated particles having a size of 1-4 mm. When these conglomerates were rubbed gently they could be broken up again into their elementary particles, which were found to be of the order of size of 5 microns and less.

Example II The above experiment was repeated with a magnesia powder (maximum diameters of 50 microns), the linear velocity of the flow of gas amounting to approximately 10 cm./sec. and the expansion of the powdered mass to The particles size of the elementary particles of the separated conglomerates was 5 microns and less.

I claim as my invention:

1. Process for segregating powdered material containing fine particles having diameters less than about 10 and coarser particles into fractions wherein finer and coarser particles, respectively, are concentrated, comprising the steps of subjecting the said material to a solely mechanical treatment for causing the finer par ticles to conglomerate by passing a gas through a layer of said material upwards at a velocity to bring said particles to a fluidized state, maintaining the particles in the said fluidized state until finer particles have formed conglomerates larger than the coarser particles, and physically separating the resulting conglomerates of finer particles from the unconglomerated, coarser particles by subjecting the resulting material to a separatory treatment which effects a separation on the basis of particle sizes.

2. The process according to claim 1 wherein the powdered material is an inorganic metal oxide powder.

3. The process according to claim 1 wherein the powdered'material is tannin powder.

4. The process according to claim 1 wherein the powdered material is magnesia powder.

5. In combination with the process of claim 1, the step of pulverizing the separated conglomerates to reconstitute the finer particles into a fine powder.

6. Process for segregating powdered material containin fine particles having diameters less than about and coarse particles having greater diameters, into fractions wherein fine and coarse particles, respectively, are concentrated, comprising the steps of subjecting said material to a solely mechanical treatment for causing the fine particles to conglomerate by passing a gas through a layer of said material upwards at a velocity to maintain said particles suspended in a dense layer of gas and powdered particles, continuing the flow of gas to maintain the particles suspended in said dense layer until fine particles have formed conglomerates larger than the coarser particles, and physically "separating the resulting conglomerates of fine particles from the unconglomerated, coarse particles by subjecting the resulting material to a separatory treatment which effects a separation on the basis of particle size".

7. Process for pre-treating powdered catalyst for fluidized operations containing fine particles having diameters less than about 10 and coarser particles for removing the finest particles therefrom comprising the steps of subjectin said catalyst to a solely mechanical treatment for causing the finer catalyst particles to conglomerate by passing an inert gas through a layer thereof upwards at a velocity to bring the catalyst particles to a fluidized state, maintaining the particles in the said fluidized state until finer particles have formed conglomerates larger than the coarser particles, and physically separating the resulting conglomerates from the unconglomerated, coarser particles by subjecting the resulting catalyst mixture to a separatory treatment which effects a separation on the basis of particle sizes.

8. Process for segregating powdered material containing fine particles having diameters less than about 10a and coarser particles into fractions wherein finer and coarser particles, respectively, are concentrated, comprising the steps or subjecting the said material to a solely mechanical treatment for causing the finer particles to conglomerate by passing a gas through a layer of said material'upwards at a velocity to bring said particles to a fluidized state wherein said particles are suspended in a dense phase of gas and powdered particles, maintaining the particles in the said fluidized state until finer particles have formed conglomerates larger than the coarser particles, and thereafter increasing the velocity of gas to classify the mixture by carrying ofi the unconglomerated, coarser particles as a gas suspension and leaving the conglomerates of finer particles.

9. Process for segregating powdered material containing fine particles having diameters less than about 10a and coarser particles into fractions wherein finer and coarser particles, respectively, are concentrated, comprising the steps or subjecting the said material to a solely mechanical treatment for causing the finer particles to conglomerate by passing a gas through a layer or said material upwards at a velocity to bring said particles to a fluidized state, maintaining the particles in the said fluidized state until finer particles have formed conglomerates larger than the coarser particles, and screening the resulting mixture of conglomerated and unconglomerated particles to produce a fraction of coarser particles passing through said screen, and a fraction or conglomerates of finer particles as a screen residue.

10. The process according to claim 9 wherein the coarser particles have diameters less than about 1 mm. and the screen has a mesh to retain conglomerates having diameter of about l-4 mm., and to pass finer, unconglomerated particles.

11. Process for segregating powdered material containing particles of different sizes some of which have diameters less than about 10 into fractions wherein particles of different fineness are concentrated, comprising the steps of subjecting the said material to a solely mechanical treatment for causin a portion of the finer particles to conglomerate by passing a gas through a layer of said material upwards at a velocity to bring said particles to a fluidized state, maintaining said particles in a fluidized state until a portion only of the finer particles have formed conglomerates larger than the coarse particles, physically separating the resulting conglomerates of finer particles from the unconglomerated particles by subjecting the resulting material to a separatory treatment which eiiects a separation on the basis of particle sizes to isolate a first fraction of fine particles, thereafter again subjecting the unconglomerated material to said solely mechanical treatment and maintaining the particles in a fluidized state until additional finer particles form conglomerates larger than the coarse particles, and physically separating the resulting conglomerates of fine particles from the unconglomerated coarser particles by subjecting the resulting material to a separatory treatment which effects a separation on the basis of particle sizes to isolate a second fraction of fine particles.

12. Process for segregating powdered material containing fine particles having diameters less than about 10 and coarse particles having greater diameters, into fractions wherein fine and coarse particles, respectively, are concentrated, comprising the steps of removing the coarsest particles from said material so that all remaining particles have diameters less than about 300;, subjecting the mixture of said remaining particles to a solely mechanical treatment to conglomerate the fine particles by passing a gas through a layer of said remaining particles upwards at a velocity to maintain said mixture suspended in a fluidized state, continuing the flow of gas to maintain thesaid mixture in a fluidized state until the fine particles have formed conglomerates larger than the coarse particles of said mixture, and physically separating the resulting conglomerates of flne particles from the googoao .,uncong1omerated coarse particles by subjecting the resulting material to a. separatory treatment 1 which eflects a separation on the basis of particle BEFERENcEs CITED The following references are of record in the 10 file of this patent:

8 UNITED s'rwrns PA'I'ENTS Number Name Date Herr Feb. 27, 1906 Reed Jan. 14, 1919 Raw July 25, 1933 Hanahan Nov. 28, 1933 Berry Feb. 24, 1942 Lefler Mar. 19, 1946 Medlin May-27, 1947 Lechthaler et a1. June 10, 1947 Lechthaler et a1 July 8, 1947 

1. PROCESS FOR SEGREGATING POWDERED MATERIAL CONTAINING FINE PARTICLES HAVING DIAMETERS LESS THAN ABOUT 10$ AND COARSER PARTICLES INTO FRACTIONS WHEREIN FINER AND COARSER PARTICLES, RESPECTIVELY, ARE CONCENTRATED, COMPRISING THE STEPS OF SUBJECTING THE SAID MATERIAL TO A SOLEY MECHANICAL TREATMENT FOR CAUSING THE FINER PARTICLES TO CONGLOMERATE BY PASSING A GAS THROUGH A LAYER OF SAID MATERIAL UPWARDS AT A VELOCITY TO BRING SAID PARTICLES TO A FLUIDIZED STATE, MAINTAINING THE PARTICLES IN THE SAID FLUIDIZED STATE UNTIL FINER PARTICLES HAVE FOMED CONGLOMERATES LARGER THAN THE COARSER PARTICLES, AND PHYSCIALLY SEPARATING THE RESULTING CONGLOMERATES FO FINER PARTICLES FROM THE UNCONGLOMERATED, COARSER PARTICLES BY SUBJECTING THE RESULTING MATERIAL TO A SEPARATORY TREATMENT WHICH EFFECTS A SEPARATION ON THE BASIS OF PARTICLE SIZES. 